A voltage controlled oscillator (VCO) or oscillator is a component that can be used to translate DC voltage into a radio frequency (RF) voltage or signal. The magnitude of the output signal is dependent on the design of the VCO circuit and the frequency of operation is determined, in part, by a resonator that provides an input signal. In general, VCOs are designed to produce an oscillating signal at a particular frequency ‘f’ that corresponds to a given tuning voltage. In particular, the frequency of the oscillating signal is dependent upon the magnitude of a tuning voltage Vtune applied to a tuning diode network across a resonator circuit. The frequency ‘f’ may be varied from fmin to fmax and these limits are referred as the tuning range or bandwidth of the VCO. The tuning sensitivity of the VCO is defined as the change in frequency over the tuning voltage and it is desirable to tune the VCO over a wide frequency range within a small tuning voltage range.
Clock generation and clock recovery circuits typically use VCOs within a phase locked loop (PLL) to either generate a clock from an external reference or from an incoming data stream. VCOs affect the performance of PLLs. In addition, PLLs are typically considered essential components in communication networking as the generated clock signal is typically used to either transmit or recover the underlying service information so that the information can be used for its intended purpose. PLLs are also important in wireless networks as they enable the communications equipment to quickly lock onto the carrier frequency on which communications are transmitted.
The popularity of mobile telephones has renewed interest in and generated more attention in wireless architectures. This popularity has further spawned renewed interest in the design of low noise wideband oscillators. In that regard, most mobile communication systems include a tunable VCO as a component in a frequency synthesizer, which selectively provides a choice of the desired channel. The recent explosive growth in the new families of cellular telephones and base stations using universal mobile telephone systems (UMTS) has stirred a need for developing an ultra-low noise oscillator with a fairly wide tuning range. The demands of wideband sources have generally increased telescopically because of the explosive growth of wireless communications. In particular, modern communication systems are typically multi-band and multi-mode, therefore requiring a wideband low noise source that preferably allows simultaneous access to DCS 1800, PCS 1900 and WCDMA (wideband code division multiple access) networks. The commercial handsets employed by these and other next generation networks are typically required to be capable of handling not only voice data; but also image and video data. Therefore, the radio link typically has to deal with signals that are more digitally complex.
Despite the continuous improvement in VCO technology, low phase noise typically remains a bottleneck and poses a challenge to RF transceiver (transmitter-receivers) design. Furthermore, oscillator/VCO design typically poses a challenge to the RF trans-receiver system. This is typically considered due to the more demanding parameters of the VCO design: low phase noise, low power consumption and wide frequency tuning range. For example, in a receiver, the phase noise of the local oscillator limits the ability to detect a weak signal in the presence of a strong signal in an adjacent channel. In a transmitter, phase noise results in energy being transmitted outside the desired channel or band.
A resonator is an important module in an oscillator circuit. MEMS (micro-electromechanical systems), SAW (surface acoustic wave) and ceramic-based resonators offer high Q factors but are relatively expensive and not easily amenable to the integrated circuit form. The Q factor of a resonator is typically considered a key factor for low phase noise performance, but the overall oscillator performance is generally controlled by the time average loaded Q of the oscillator circuit. Ceramic resonator-based oscillators are used in wireless applications, and typically provide low phase noise at fixed frequencies. Unfortunately, ceramic oscillators have disadvantages including a relatively limited operating temperature range, tuning range (which limits the amount of correction that can be made to compensate for the tolerance of other components in the oscillator circuitry) and an increased sensitivity to phase hits in the PLL.
In addition, oscillator circuitry becomes susceptible to phase-jitter and phase-hits due to noise-interference that is typically present on the tuning control voltage and resonator network. A VCO that outputs a frequency that is less sensitive to tuning control voltage fluctuations imparts less phase jitter to the system, but suffers from a narrower tuning range, which limits its tuning band. In applications where coarse-tuning and fine-tuning is required, phase noise performance typically suffers in the presence of a fine-tuning network that is incorporated with a coarse tuning network. Further, the fine-tuning network typically adds extra noise and loading to the resonator circuit. Thus, the performance of the tuning circuit network is becoming even more important.
FIG. 1A, for example, depicts a typical prior art oscillator 100. The oscillator includes an active device 103 or Q1 having a collector terminal 1031, a base terminal 1032 and an emitter terminal 1033. A frequency selective network is provided via a micro-stripline resonator 107 that is capacitively coupled via capacitor Cc1 to the base terminal 1032. The micro-stripline resonator 107 is also capacitively coupled to a tuning network 110 via capacitor Cc2. A voltage source Vt is coupled to the tuning network 110 via a resistor 114. A biasing network 120 is coupled to the collector terminal 1031. A first feedback capacitor C1 is connected between the base and emitter terminals, 1032 and 1033 respectively. The capacitor C1 is also coupled to capacitor C2, which is grounded. The frequency of the resonator 107 is tunable over a frequency band by adjusting the voltage output of the source Vt. This results in the operating or oscillation frequency of the oscillator 100 to be tuned over an output frequency band, which is detectable at the terminal labeled Pout.
In the oscillator of FIG. 1A, the micro-stripline resonator generally needs to have a relatively low impedance for a high Q factor over the desired tuning range. The resonator may be produced by an etching in a micro-stripline. Therefore, the oscillator frequency is affected by the tolerance values of the components used without generally having an effect on the resonator frequency. However, micro-stripline resonator-based oscillator circuitry typically exhibits a high degree of sensitivity to changes in the surrounding environment causing them to become microphonic and sensitive to noise interference. Stripline resonators are less sensitive to microphonic and other types of interference, since they are self-shielding due to their dual ground plane structure. However, choosing a stripline based resonator generally results in a lower Q (for same physical dimension as compared to a micro-stripline) and higher capacitance at given frequency range. In addition, micro-stripline or stripline resonators typically take up valuable space on the oscillator's circuit board. Therefore, there is a need to retain the higher Q's associated with the micro-stripline, while providing the additional shielding against interference, in particular microphonic interference, available in a stripline, as well as minimizing the amount of board area required to implement the micro-stripline resonator.
FIG. 1B shows a schematic of a prior art oscillator that operates as a hybrid-tuned (coarse/fine) ultra low noise wideband voltage controlled oscillator. Normally, a fine-tuning network 150, as shown in the FIG. 1B, adds extra noise and loading to the resonator circuit 107, which in turn makes the tuning networks 110, 150 more critical. Circuits of the type shown in FIG. 1B normally exhibit poor phase noise performance over the desired tuning range (e.g., 1200-3600 MHz). In addition, the tuning range is limited because of the stability factor of the negative resistance-generating device, i.e., the transistor Q1, over the frequency band.
Of utility then are methods, apparatus and systems that provide a low noise wideband VCO using stripline or micro-stripline technology. Of additional utility are methods, apparatus and systems that provide an ultra-low noise, hybrid tuned and power-efficient wideband VCO.